Alkaline earth metal oxides, especially magnesium oxide, are highly useful industrial chemicals. They are used in flue gas desulfurization, wastewater neutralization and as a raw material for the manufacture of other alkaline earth metal compounds. In particular, magnesium oxide is employed as a raw material for the manufacture of other magnesium compounds. It is highly desired to have an improved method for preparing magnesium hydroxide by the hydration of magnesium oxide with water.
G. L. Smithson & N. N. Bakhshi, The Kinetics and Mechanism of the Hydration of Magnesium Oxide in a Batch Reactor, 47 CANADIAN JOURNAL OF CHEMICAL ENGINEERING 508-513 (October 1969), disclosed a study on the hydration of magnesium oxide and reported therein that it appeared to be a simple heterogeneous reaction and an intermediate step in the carbonation of aqueous MgO slurries. Smithson discloses that the hydration of MgO samples was carried out by adding MgO to a reactor containing distilled water. About 10 grams of MgO was reported to have been reacted with 250 milliliters of distilled water. A stirrer speed was maintained at 1600 revolutions per minute indicating agitation was employed in the reactor. According to Smithson, the size of the MgO powder fed to this reaction apparently was of the order of micro-sized particles as Smithson refers to micro-sized particles in column one, page 509 prior to disclosing the hydration process. Smithson noted that his MgO particles were apparently reactive. Smithson also discloses that the reactivity of MgO powders is dependent on surface area and calcination temperature and teaches that higher relative reactivity of MgO particles is obtained when one uses MgO with a high surface area and which has been calcined at a temperature below about 1200.degree. C.
Japanese Laid-Open Patent Application (Kokai) No. 1-212214, published Aug. 25, 1989 (English text translation), discloses a method for manufacturing magnesium hydroxide where reportedly magnesium oxide with an average particle diameter of 100 .mu.m or less can be hydrated in an aqueous medium in a stirred reactor (agitation) containing (a) one or more types of cations selected from the group consisting of alkali metal ions, alkaline earth metal ions and ammonium ions and (b) one or more types of anions selected from the group consisting of hydroxide ions, nitric acid ions, carbonic acid ions, chlorine ions and sulfuric acid ions. This published patent application teaches that the magnesium oxide must have an average particle diameter of 100 .mu.m or less, with 50 .mu.m or less being preferable and 20 .mu.m or less being even better. This published patent application discloses that if the average particle diameter of the magnesium oxide exceeds 100 .mu.m, the suspension stability of the magnesium hydroxide slurry will be poor and there will be problems with the unreacted residue increasing the amount of scaling and the suspended solids load.
Magnesium oxide (magnesia, MgO) is the principal product of the magnesium compounds industry. Further, according to 14 KIRK-OTHMER, ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY: Laminated Wood-Based Composites to Mass Transfer (John Wiley & Sons, 3rd Ed. (1981)), magnesium occurs as the mineral periclase in nature. But, magnesium also occurs naturally in brines, seawater, magnesite and dolomite and also it occurs as disseminated crystals or clusters of crystals in marbles. The principal properties of magnesium oxide are provided in the 14 ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Table 12, pp. 629-646, all of which is incorporated herein by reference in its entirety.
Commercial magnesium oxide is available as caustic calcined magnesia, dead-burned magnesia, hard-burned magnesia and fused magnesia. As described in ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, supra, and other technical sources, dead-burned magnesias are produced above 1400.degree. C. and are characterized by their low chemical reactivities and their resistance to the basic slags employed in the metals refining industry. ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, supra, teaches that, typically, the sizes range from about 30 .mu.m to about 100 .mu.m in diameter. Magnesium oxide is light, has a high melting temperature and is brittle.
The process of this invention overcomes the low chemical reactivity of MgO calcined at temperatures in excess of 1200.degree. C., e.g. and provides a process which effectively accommodates a far wider range of particle sizes of magnesium oxide and avoids any need to grind or reduce purchased or intact MgO to powder size before hydration can be carried out. This invention can be carried out in a quiescent state and without the addition of cations.